1. Field of the Invention
This invention relates generally to high-frequency amplifier circuits employing a transistor or the like element mounted on a substrate having microstrip lines and, more particularly to those of the type including a biasing circuit in the form of a quarter-wavelength microstrip line.
2. Description of the Prior Art
A conventional high-frequency amplifier circuit usually includes a transistor or the like amplifier element mounted on a substrate having microstrip lines, microstrip-line-type impedance matching circuits coupled, respectively, to the input and output sides of the amplifier element, and a biassing circuit. As is known, the biassing circuit is formed, for example, of such a choke coil as having no influence on the matching circuits (see U.S. Pat. No. 3,869,678) or of a line substantially of the length of a quarter wavelength having a characteristic impedance higher than the impedance of the matching circuit. A biassing circuit employing a choke coil is advantageous in that it involves only limited high-frequency and bias-current losses and enables increase in bandwidth of the circuit but is disadvantageous in that it can hardly be reduced in size as the choke coil can hardly be made of microstrip from and that it involves high cost, including many discrete components. On the other hand, a biassing circuit of the quarter-wavelength line type involves only a limited labor cost for fabrication as it can be formed efficiently on a substrate simultaneously together with a wiring pattern, matching circuits and others and, unlike the choke coil, does not necessitate any discrete components, thus enabling further reduction in cost. In cases, however, where the quarter-wavelength line is formed of a microstrip line by the inexpensive, thick-film printing technique, the width of the strip line cannot be reduced to any extreme extent thereby to increase the characteristic impedance of the line because of the manufacturing accuracy of the thick film technique or the loss of bias current involved. This means that the microstrip line has a substantial influence on the matching circuits to limit any increase in frequency range or frequency bandwidth of the amplifier circuit.